In the preparation of composite panels for building construction comprising a cementitious facing material adhered to a generally smooth skinned foam material such as a styrene polymer foam, difficulty is experienced in obtaining an adequate and permanent bond between the contacting surfaces of the dissimilar materials. This is due at least in part to the fact that the prior known cementitious materials in setting, tend to shrink whereas the foam surface with which it comes in contact does not undergo a corresponding shrinkage. Also, the differences between the coefficients of thermal expansion between the two types of materials often provides a severe shearing stress which results in failure of the bond between the materials.